1. Field of the Invention
The present invention relates to a method for measuring optical characteristics and a system thereof in an optical communication system for transmitting WDM (Wavelength Division Multiplexed) light.
2. Description of the Related Art
In recent years, a function for automatically measuring and displaying optical loss (i.e., a span loss) in an optical communication system between terminals, between a terminal and a relay station or between relay stations has been requested in order to improve user operability and in order to automate various settings. This measurement function includes a function that will notify a user of a change in span loss caused by external factions during operation of the communications system. Additionally, during the initial start-up of the communication system or the like, the measurement function is also used for measuring error between a designed value (determined in advance by simulation or similar processes) and actual span loss, or used for confirming whether the input level/output level of an optical transmission system has a predetermined provision.
The span loss between stations is, in principle, determined by providing a photo detector for detecting optical power in the transmitting station and the receiving station respectively and calculating the difference between the optical output level in the transmitting station and an optical input level in the receiving station. Note that the patent document 1 (Japanese Patent Application Laid-Open No. 9-116504) discloses a configuration in an optical communication system provided with two-system optical fiber transmission lines for guiding a part of an optical signal propagating through one of the transmission lines to the other transmission line and measuring the power of the optical signal so as to measure the gain and the loss in the optical fiber transmission system.
In a WDM transmission system, a plurality of optical signals are transmitted using a plurality of different wavelengths. In addition, many span loss standards and optical level of the transmission line standards are specified as characteristics at a central wavelength of a predetermined wavelength band, including the signals (e.g., approximately 1550 nm for C-band). Therefore, it is desirable that the span loss (or the like) of the transmission line may be measured using an optical signal which possesses the central wavelength during the initial start-up of the transmission system or during the operation of the transmission system.
However, in practice, there is no determination of which of the plurality of wavelengths that have been prepared in advance is to be used. That is to say, in some cases, the central wavelength that is used in designing the transmission system is not actually used. On the other hand, a loss in the optical fiber has wavelength dependence as shown in FIG. 1. Accordingly, it is difficult for the customer to accurately know the span loss at the central wavelength, which the customer needs to know in the case in which a wavelength that is different from the central wavelength is used. Additionally, during the initial start-up of the system, it may be assumed that no single wavelength within the WDM signal wavelength band will be inputted. In that case, as well, the span loss is not measurable.
One possible solution to this daunting problem is a configuration which uses an OSC (Optical Supervisory Channel) signal. In general, an OSC signal is allocated a wavelength that is out of the signal wavelength band for transmitting the data, and the OSC signal is always transmitted during operation of the transmission system. According to this method, the optical level of the OSC light carrying the OSC signal is measured in the transmitting station and the receiving station to measure the span loss at the OSC wavelength. Next, the span loss at the central wavelength is estimated from the span loss at the OSC wavelength based on the predetermined wavelength characteristics of the optical fiber. According to this method, the span loss at the central wavelength can be estimated without being affected by the number or the allocation of wavelengths that are actually used. However, the information representing the distance between the stations (i.e., the length of the optical fiber) is required in order to estimate the span loss at the central wavelength from the span loss of the OSC light. Therefore, if accurate information about the distance between the stations is not available, the estimated span loss may have significant error.